At the present time, Liquid Crystal Display (LCD) projection systems using LCD panels are widely used in large screen television (over 32 inches diagonal) and in stand-alone projectors controlled by a computer, such as a PC (Personal Computer). Such projection systems are widely used for presentation and entertainment by businesses, teachers and consumers.
In an LCD projector the light from the "light engine" is directed, by a lens system, through one, or three, LCD panels. A typical light engine consists of a light source, such as a metal halide arc lamp, a collector optic, such as an elliptical mirror reflector, and one or more lenses to direct the light onto the "image gate", which is the LCD panel (plate). It is desirable that the light engine have the following characteristics: (1) a bright, efficient light source; (2) low heat generation for effective cooling and to prolong the life of the lamp; (3) white light without blotching; (4) light spread evenly on the screen with the corners being at least 60% as bright as the center; and (5) physically small and low cost. The key to a good and low cost design is to maximize the efficiency of the throughput of the light emitted by the lamp. The more lumens on the screen, compared to the per watt of power of the lamp, the better the design. An active matrix LCD consists of tiny picture elements ("pixels") which are switched on and off. An organic fluid, called a "liquid crystal", is held between transparent plates. Generally the crystals are transparent but can alter the orientation of polarized light passing through them when the alignment of their molecules is changed by applying an electrical field across the crystals.
In a color LCD panel made of a single plate, the two outside faces of the transparent plate are coated with a polarizing filter (sheet polarizer) so that only P (parallel to plane of incidence) or S (perpendicular to plane of incidence) directed light waves may pass. Each full color pixel comprises a red, green and blue subpixel which has color filters so that only red, green or blue light is transmitted.
In a normally open panel, when no power is applied, light incident on the first polarizer is plane polarized along a chosen plane. The liquid crystals, with no power on, are aligned to twist the polarized light through 90.degree.. The second polarizer/analyzer is set at 90.degree. to the first one. In this manner, light is transmitted along a single polarization plane through the panel when there is no power on. Most LCD projection panels are of this type. When a pixel (or sub-pixel) is activated through the application of an electric field (power on), the polarized light will not be twisted by 90.sqroot. by the Liquid Crystal and will therefore be blocked by the second polarizer/analyzer.
The active matrix consists of one transistor for each subpixel, formed directly thereon, and the connecting printed wires. The wires are generally formed in column addressing lines and in row addressing lines.
The polarized light is derived from a non-polarized light source, such as a bulb. Due to the filtering, only one-half of the light output of the light source is utilized.
It is desirable, in many LCD projector systems, that all or most of the light be utilized. This would result in a brighter picture, using the same size of bulb. Alternatively, the bulb size may be reduced, which reduces the heat generated by the light engine. A smaller bulb may be cooler and may have a longer life. It has been a major goal in the LCD projection industry to develop ways to recover the part of the light in a projection system that is not polarized in the required polarization plane. Such light is lost from the projected beam. If successful, such a device, called a "polarizer doubler" will double the overall light efficiency of a projection system to greatly improve a projector's performance-to-cost ratio.
A number of prior patents and articles have suggested that the unpolarized light may be separated into two polarized beams, the polarization of one of the beams reversed and the two beams combined. That type of system is called a "polarizer doubler" as it doubles the amount of light available in one polarization. Such a polarizer doubler generally uses a Polarizing Beam Splitter (PBS) which separates light into its two polarizations.
In U.S. Pat. No. 4,913,529 to Goldenberg et al, an incident beam of unpolarized light is directed to a polarizing beam splitter (PBS) which reflects a beam of S-polarization (perpendicular to plane of incidence) and passes a beam of P-polarization (parallel to plane of incidence). The S-polarization beam is directed through a polarization rotator (half-wave retarder plate) which rotates the beam 90.degree.. The two beams are then combined using a prism.
U.S. Pat. No. 5,601,351 and European Patent Application 0467-447-A1 to Brandt disclose a polarizer doubler for an image projection apparatus. It uses a polarization-sensitive beam-splitting prism and a polarization rotator which is a birefringent adhesive layer on a face of the prism. An optical integrator, positioned after the polarizer doubler, may include a "light guiding tube" (light pipe) whose entrance face is adapted to the cross-section of the beam emerging from the polarizing system and whose exit face is adapted to the shape and dimensions of the object to be illuminated. Brandt states that preferably the optical integrator is a first and second lens plate. Claim 1 of the U.S. Patent states that the polarizing system is between the optical integrator and the light source.
A portion of the prism and system of the Brandt patent is described in the article, DeVaan, Brandt et al, "Polarization conversion system LCD projection", Euro-Display 1995, pgs. 253-256.
U.S. Pat. No. 5,278,680 to Karasawa et al discloses a polarizing beam splitter (PBS) between the light source and the LCD plate. A polarization conversion device (polarizer rotator) converts the polarization of one beam. The two beams are reflected by mirrors and are superimposed, in one embodiment, in a prism to form a single beam. FIGS. 11 and 12 show a cube prism used to reflect one polarization and transmit the other polarization.
In U.S. Pat. No. 5,513,023 to Fritz a polarizing doubler transmits light from a light source to PBS using two fiber bundles (fiber optic cable) and collimating lenses.
Other polarizer-doubler systems for LCD panels are disclosed in U.S. Pat. No. 4,798,448 to van Raalte; U.S. Pat. No. 5,566,367 to Mitsutake et al.; U.S. Pat. No. 5,653,520 to Kato; U.S. Pat. No. 5,485,310 to Inada; and U.S. Pat. No. 4,702,557 to Beckmann and in the following articles: Nicholas et al., "Efficient Optical Configuration for Polarized White Light Illumination of 16/9 LCDs in Projection Display", Japan Display '92, pgs. 121-124; Shikama et al., "A Polarization Transforming Optics For High Luminance LCD Projector", Japan Display '93, pgs. 26-29; Imai et al., "A novel polarization converter for high-brightness liquid crystal light valve projector", Euro Display '93, pgs. 257-260 and Japan Display '93, pgs. 235-237.
U.S. Pat. No. 5,625,738 does not relate to a polarizer doubler. In FIGS. 2a-3 it shows a light tunnel (light pipe) which is tapered for a projection system.
The above-cited patents and articles are incorporated by reference.